Field of the Invention
The present invention relates to an image forming apparatus which includes a rotary polygon mirror.
Description of the Related Art
Hitherto, it has been generally known that electrophotographic image forming apparatus such as a laser printer and a copying machine include a light scanning apparatus configured to scan a photosensitive drum with a light beam to form an electrostatic latent image on the photosensitive drum. The light scanning apparatus includes a light source configured to emit a light beam, a collimator lens configured to collimate the light beam emitted from the light source, a rotary polygon mirror configured to deflect the collimated light beam, and an fθ lens configured to form an image of the deflected light beam on the photosensitive drum. In order to fix a writing start position of the electrostatic latent image in a main scanning direction on the photosensitive drum, the light beam deflected by the rotary polygon mirror is detected by a beam detector, and an emitting start timing of the light beam is determined based on a detection timing of the light beam by the beam detector as a reference.
The rotary polygon mirror has a plurality of reflection surfaces. The reflection surfaces may have different angles and degrees of flatness due to manufacture tolerance. Thus, when the emitting start timings of the light beam are determined based on a detection result of the beam detector configured to detect the light beam deflected by the reflection surfaces of the rotary polygon mirror, there is a problem in that fluctuation in angles and degrees of flatness of the reflection surfaces may cause deviations of writing start positions of the electrostatic latent images to be formed on the photosensitive drum. The deviations of writing start positions caused by the tolerance in angles and degrees of flatness of the reflection surfaces of the rotary polygon mirror may occur repeatedly in one rotation cycle of the rotary polygon mirror.
To cope with such a problem, in Japanese Patent Application Laid-Open No. 2004-271691, there is disclosed that the emitting start timing of the light beam for each reflection surface of the rotary polygon mirror is stored in advance to control the light source based on the emitting start timing stored for each reflection surface based on the detection timing of the light beam by the beam detector as a reference.
However, there is a case where the light scanning apparatus is deformed due to a rise in internal temperature of the image forming apparatus, with the result that a position of the beam detector is shifted. The deviation of position of the beam detector causes a problem in that a writing start position for an electrostatic latent image to be formed on the photosensitive drum is shifted even when the light source is controlled based on the emitting start timing for each reflection surface stored in advance at the time of factory shipment.
FIG. 20A and FIG. 20B are views for illustrating positions of spots SP1 and SP2 of light beams LB1 and LB2 which enter the beam detector 207. In each of FIG. 20A and FIG. 20B, there are illustrated the spot SP1 of the light beam LB1 deflected by a first reflection surface of the rotary polygon mirror and the spot SP2 of the light beam LB2 deflected by a second reflection surface of the rotary polygon mirror. FIG. 20A is an illustration of positions of the spots SP1 and SP2 of the light beams LB1 and LB2 when the beam detector 207 is arranged at an ideal position. FIG. 20B is an illustration of positions of the spots SP1 and SP2 of the light beams LB1 and LB2 when the beam detector 207 is inclined from the ideal position.
The light beam LB1 and the light beam LB2 illustrated in each of FIG. 20A and FIG. 20B are shifted in a sub-scanning direction which is perpendicular to a main scanning direction of the light beams LB1 and LB2. The positional deviation of the light beam LB1 and the light beam LB2 in the sub-scanning direction is mainly caused by fluctuation (hereinafter referred to as “optical face tangle error”) of angles of the reflection surfaces with respect to a rotary axis of the rotary polygon mirror due to the manufacture tolerance.
Even when scanning positions of the light beams LB1 and LB2 are shifted in the sub-scanning direction due to the optical face tangle error, timings of entry of the light spots SP1 and SP2 to a light receiving surface 208 of the beam detector 207 are fixed irrespective of the reflection surfaces as long as the beam detector 207 is arranged at the ideal position of FIG. 20A. Meanwhile, when the beam detector 207 is arranged with inclination as illustrated in FIG. 20B, the deviation of scanning positions of the light beams LB1 and LB2 in the sub-scanning direction due to the optical face tangle error causes changes in timings of entry of the light spots SP1 and SP2 which enter the light receiving surface 208 for the reflection surfaces. The inclination of the beam detector 207 as illustrated in FIG. 20B causes a deviation of timings of entry of the light spots SP1 and SP2 to the light receiving surface 208 by time T for a first reflection surface and a second reflection surface. The shift time T in timings of entry causes a deviation of writing start positions of the electrostatic latent images to be formed on the photosensitive drum for the first reflection surface and the second reflection surface.
As described above, the deviation of writing start positions of the electrostatic latent images for the reflection surfaces may cause cyclical uneven density in an image. Such an inclination of the beam detector 207 is caused by deformation of the light scanning apparatus due to the rise in internal temperature of the image forming apparatus, with the result that the deviation of writing start positions of the electrostatic latent images occurs even when a mounting position of the beam detector 207 is adjusted at the time of factory shipment. Further, when the light scanning apparatus is deformed due to the rise in internal temperature of the image forming apparatus, in addition to the inclination of the beam detector 207, there are also changes in mounting angles of optical components such as reflection mirrors and lenses configured to guide the light beam from the reflection surfaces of the rotary polygon mirror to the beam detector 207. The changes in mounting angles of the optical components such as the reflection mirrors and lenses may also cause the deviation of writing start positions of the electrostatic latent images as with the inclination of the beam detector 207.
Even when the light source is controlled based on the emitting start timings of the reflection surfaces stored in advance at the time of factory shipment as disclosed in Japanese Patent Application Laid-Open No. 2004-271691, the deviation of writing start positions of the electrostatic latent images cannot be corrected satisfactorily in the case where the position of the beam detector is changed from an initial position due to the rise in internal temperature of the image forming apparatus.